Nitrobenzene has been known to have a toxic effect upon the hemopoietic system. One of the major pathological changes observed with nitrobenzene administration in acute studies is methemoglobinemia. In order to understand the mechanism of formation of methemoglobin, we have studied the relationship of biotransformation of nitrobenzene to the toxicity. This investigation will further serve as a model study for more complex molecules. In particular, these studies may serve as an important model for studying various toxicities produced by the antibiotic chloramphenicol which contains a p-nitrophenyl group.